1. Field of the Invention
This invention relates to a method for the reduction of acidity in drainage from pyritic mine tailings, coal mines, coal refuse piles and construction sites.
2. Description of the Prior Art
Acid drainage results from the oxidation of iron pyrite (FeS.sub.2) associated with coal, mine tailings and overburden material. The problem of acid formation also may occur during building or road construction due to pyrite present in the disturbed strata. The conventional technique to overcome this problem has been to collect the drainage from the area and then neutralize it with lime and/or limestone.
Previous research by biologists suggests that a bacterium, Thiobacillus ferrooxidans, plays a role in the acid formation process. However, geologists have remained sceptical of such research primarily because the laboratory experimental conditions tend to deviate radically from actual field conditions. Hence any correlation between laboratory and field conditions is suspect.
The tolerance of T. ferrooxidans to a large range of chemical substances has been examined in the literature. Indeed, two detergents, alkyl benzene sulfonate and sodium lauryl sulfate, were tested by Dugan and Lundgren in 1964 and shown to inhibit T. ferrooxidans at concentrations of 2-5 ppm in culture media. Another detergent was also tested and found inhibitory at only extremely high concentrations. The inhibition observed was attributed by Dugan and Lundgren to a coating of the bacteria caused by the detergents' alkyl side chain.
It is of course possible to protect coal from contact with water and thereby prevent formation of acid drainage by coating each individual particle with various oils and waxes. Moreover, coal may be protected as disclosed in U.S. Pat. No. 2,204,781, Wattles, by spraying the outside surface of piles or large masses of coal with a material congealable at atmospheric temperatures whereby a non-tacky weather resistant jacket adherent to the coal at the surface of the pile or mass is formed. However, both of the foregoing procedures require substantial expenditures of coating material, and are not practical where the primary purpose is to avoid acid drainage as opposed to the prevention of water contact.
One prior art approach to prevention of acid drainage relates to treatment with an aqueous alkali metal silicate solution which reacts with acid present to form a sila gel which in turn may coat the pyrite surface. (U.S. Pat. No. 3,094,846). Another prior art method of preventing acid drainage involves use of a mixture of pulverized phosphate and sulfuric acid to precipitate soluble iron as phosphates which in turn may coat the pyrite surface, (U.S. Pat. No. 3,443,882).
In accordance with this invention acid drainage in a pyritic material which is exposed to water is controlled by addition of at least about 10 ppm of an anionic detergent capable of inhibiting T. ferrooxidans to the water which contacts the exposed material. The concentration is necessarily higher than that previously reported to be required to inhibit T. ferrooxidans in culture media. In order to avoid repopulation, the addition is effected by controlled release from a matrix containing anionic detergent dispersed therein. Controlled addition is accomplished by placement of the matrix to intercept infiltrating water upgradient of the pyritic material. The quantity of anionic-detergent containing matrix required to effect the controlled release is calculated in accordance with the formula set forth in the summary of the invention.
I have developed a method of dealing with acid drainage which, unlike the conventional prior art, does not address the problem by treating the resultant acid, but rather prevents acid formation in a facile and economically viable manner.